This invention relates to high strength nickel-iron aluminide alloys that exhibit desirable hot ductility and fabricability.
Ordered intermetallic alloys based on tri-nickel aluminide (Ni.sub.3 Al) have unique properties that make them attractive for structural applications at elevated temperatures. They exhibit the unusual mechanical behavior of increasing yield stress with increasing temperature whereas in conventional alloys yield stress decreases with temperature. Tri-nickel aluminide is the most important strengthening constituent of commercial nickel-base superalloys and is responsible for their high-temperature strength and creep resistance. The major limitation of the use of such nickel aluminides as engineering materials has been their tendency to exhibit brittle fracture and low ductility.
Recently alloys of this type have been improved by the additions of iron to increase yield strength, boron to increase ductility, and titanium, manganese and niobium for improving cold fabricability (Commonly assigned and co-pending U.S. patent application Ser. No. 519,941 filed Aug. 3, 1983, Ductile Aluminide Alloys for High Temperature Applications, Liu and Koch). Another improvement has been made to the base Ni.sub.3 Al alloy by adding iron and boron for the aforementioned purposes and, in addition, hafnium and zirconium for increased strength at higher temperatures (Commonly assigned and co-pending U.S. patent application Ser. No. 564,108 filed Dec. 21, 1983, U.S. Pat. No. 4,612,165, Ductile Aluminide Alloys for High Temperature Applications, Liu and Steigler). These co-pending U.S. patent applications are incorporated herein by reference.
Although these improved alloys have many beneficial characteristics, they still exhibit some shortcomings which detract from their usefulness. For example, the previous nickel aluminide alloys suffer a decrease in ductility and workability with increasing temperature. Any fabrication of the alloys into structures of desired configurations by rolling or forging must be achieved at temperatures less than 700.degree. C. Such alloys would be of greater value if the hot fabricability could be achieved at a higher temperature of up to about 1,200.degree. C. since industry fabrication experience and capability exist at this temperature. Other benefits derived from fabrication at higher temperatures include reduction in the fabrication cost and the elimination of the need for high-power fabrication equipment.